1. Field of the Invention
This invention relates to a novel cutting and lubricating composition for use with an apparatus for cutting workpieces of hard and brittle material such as semiconductor ingots of silicon, germanium, gallium arsenide or glass or other brittle materials, such as granite block, or ceramics or steel parts into a multiplicity of thin sheets, slices, wafers, or precision machined parts with abrasive particles which are kept in suspension.
2. Description of the Prior Art
For one of the major applications of the invention, the cutting apparatus, referred to as a xe2x80x9cwiresawxe2x80x9d or xe2x80x9cwire-webxe2x80x9d, usually comprises a row of fine wires arranged parallel to each other and at a fixed pitch. A workpiece is pressed against these fine wires having diameters in the order of 0.15-0.2 millimeters running in parallel with one another in the same direction, while an abrasive suspension fluid is supplied between the workpiece and the wires, thereby slicing the workpiece into wafers by an abrasive grinding action. The liquid suspended abrasive particles are coated onto the moving xe2x80x9cwebxe2x80x9d or wire through a circulation system which drops a xe2x80x9cblanket-curtainxe2x80x9d of the abrasive suspensions onto the xe2x80x9cwebxe2x80x9d just before the wire-web impacts the workpiece. Thus, the abrasive particles carried by the liquid are transferred via the coated wires to produce a grinding or cutting effect. The above described splitting units or machines, called wiresaws, are described in U.S. Pat. Nos. 3,478,732, 3,525,324, 5,269,275 and 5,270,271, which are incorporated by reference.
The cutting apparatus may also comprise a series of wires inter-looped or entwined together in a braided loop configuration. This configuration can be used for the cutting of granite block or silicon ingots. The workpiece is pressed against the braided wire and the cutting process is augmented by the abrasive particles as described above.
Achieving an optimum cutting quality depends on a combination of parameters, the quality of the abrasive fluid and the force with which the workpiece is pressed against the set of abrasive coated wires.
Effort is now directed to optimizing the cutting quality obtained under mass production conditions. By cutting quality is meant exact planarity of the surfaces without taper and thickness variation to yield products suitable for semiconductor devices, solar cells, and optical glass, among others. Mass production considerations, for example, the rate of wear of the wire, the effectiveness recovery and recycling of the cutting and lubricating fluids are also important. U.S. Pat. No. 5,099,820 issued to Stricot discloses an abrasive liquid as a suspension of particles of silicon carbide in water or oil. However, these prior art suspensions are not stable and do not provide uniform coating on the xe2x80x9ccuttingxe2x80x9d wires. Furthermore, these compositions require vigorous agitation to maintain uniform suspension of the particles, and the suspension settles out quickly under stagnant conditions, and even during workpiece slicing while still under agitation.
Thus, there exits a need for a novel cutting and lubricating composition which provides a uniform supply of homogeneously dispersed abrasive material without abrasive particle agglomeration or xe2x80x9chard-cakexe2x80x9d formation from suspension fall-out so that the workpiece is more efficiently cut by the abrasive grains or grit in the composition. Further, the composition should have excellent lubricity and heat transfer properties to remove the frictional heat generated at the cutting site thereby increasing working life of the wire and avoiding downtime. Finally, the composition should provide a stable suspension of abrasive particles. However, if stored for a long period and separation occurs, only a gentle agitation should be required to restore the suspension.
The role of surfactants and electrolytes or polyelectrolytes in the preparation of suspension concentrates is described by the effect on the wetting of the powder by a liquid, breaking the aggregates and comminution of the resulting particles into smaller units. The role of the surfactant and polyelectrolyte in controlling colloidal stability is described in the interaction forces between the particles. These interaction forces are classified into electrostatic, van der Waals and steric repulsion. With polyelectrolytes, stability is primarily the result of a combination of electrostatic and steric repulsion.
According to the broadest aspect, the present invention relates to a cutting and lubricating composition for use with an apparatus for cutting workpieces of a hard and brittle material such as semiconductor materials, magnetic materials, ceramics, granite block, solar energy components, and the like. Further, such compositions are effective and useful for the precision cutting of metal or ceramic parts as components for tools, automotive, machine or other type devices. Other applications of this invention can be easily conceived by those skilled in the art when the suspension benefits of this invention provide advantageous performance results, such as in the grinding of hard substrates. More specifically, the lubricating composition, which may contain up to 70% (wt/wt) of an abrasive material preferably comprises:
a) from about 0.0 to 10 weight percent of an ionized surfactant;
b) from about 86 to 99.5 weight percent of a non-aqueous, non-ionic, polar solvent such as polyalkylene glycols or the co-glycols thereof, and
c) an organic electrolyte or polyelectrolyte with a repeating chain pendant group having the same charge on the repeating chain pendant group as said surfactant and present in an amount to cause a sufficient electrostatic repulsive force between said abrasive particles and the surrounding liquid medium containing said polyelectrolyte to prevent both hard-cake settling and particle agglomeration of said particles when such particle suspension is subjected to stagnant storage over an extended time period at ambient or elevated temperature. Generally about 0.05 to 10.0% by weight of said polyelectrolyte is utilized, preferably between about 0.5 to 4% by weight of the carrier composition.
Advantageously, less than 10% by weight of water is present.
Thickeners may be added to the composition, which includes carboxylmethylcellulose, methylcellulose, polysaccharides, and the like, familiar to those skilled in the art. Additionally or optionally, dispersing solvents or suspension agents such as those described in U.S. Pat. No. 6,054,422 and including but not limited to, tripropglene glycol methyl ether, DPM, NMP, DMAC, DAM, and the like can be added as needed for viscosity adjustment, better slurry rinseability, etc.
A preferred substantially non-aqueous lubricating/suspension composition comprises:
a) from about 0.0 to 1 weight percent of an anionic surfactant;
b) from about 0.1 to 4 weight percent of an anionic organic polyelectrolyte or electrolyte,
c) from about 91 to 99.5 weight percent of a polyethylene glycol solvent, and
d) from about 0.0 to about 5% percent water, with the proviso that the electrolyte or polyelectrolyte is in the neutralized form ranging in pH from about 4.5 to about 8.0, preferably less than 7.
Most preferably, according to the present invention is provided a lubricating carrier composition which comprises:
a) about 93.5 to 99 weight percent of polyethylene glycols wherein said polyethylene glycols consist of a molecular weight of from about 200 to 600; and whereby the viscosity of the composition is about 50-300 cps under room temperature conditions (25xc2x0 C.),
b) about 0.1 to 0.5 weight percent of a fluorinated anionic surfactant,
c) about 0.4-3.0 weight percent of an anionic polyelectrolyte neutralized to a pH of about 4.5-6.0, and
d) about 0.5-5.0 weight percent of water
It is therefore a primary object of this invention to provide a lubricating/suspension composition for a xe2x80x9cmulti-wirexe2x80x9d machine for cutting a hard and brittle material which increases production efficiency and quality of the sliced sheet or wafer.
Normally about 40 to 50% by weight of abrasive particle material is dispersed and suspended in the carrier/lubricating composition.
It is another object of this invention to provide a cutting and lubricating composition which allows for the uniform distribution of the abrasive material to the coating of the cutting wire.
It is a further object of this invention to provide a lubricating composition which is fully water soluble, water miscible and of very low toxicity.
Yet another object of the invention is to provide a cutting and lubricating composition wherein the abrasive cutting material is suspended in the composition and remains suspended without hard-cake formation or particle agglomeration even over long periods of stagnant storage.
Yet a still further object is to provide high quality sliced sheets or wafers suitable for semiconductor and solar devices.
Other objects and applications for the composition invention and a more complete understanding of the invention will be had by referring to the following drawings and description.